Radio frequency signals transmitted by satellites (for example, geosynchronous global navigation satellites) to receivers on the Earth's surface are delayed as they travel through the Earth's ionosphere. Accordingly, attempts are made to estimate and correct for these delays. Global positioning system (GPS) augmented systems provide users with ionosphere corrections for single-frequency measurements of the GPS signal. Space based augmentation systems (SBAS) support regional global navigation satellite systems (GNSS) augmentation through the transmission of additional satellite-broadcast messages. In these systems, a network of dual frequency GPS receivers at wide area reference stations estimate delays imparted by the ionosphere along the line of sight of each receiver. These estimated delays are relayed to and broadcast by the geosynchronous satellites. Interpolation of these measurements to a predefined set of grid nodes, referred to as ionospheric grid points (IGPs), at a designated height of 350 kilometers (km) above sea level provides a series of ionospheric delay estimates for the user. The ionospheric information is broadcast to single-frequency SBAS users based on the mutually agreed ionospheric model of grid vertical total delay values, called grid ionospheric vertical delays (GIVD). In addition, some SBAS systems also provide an indication of GIVD accuracy, known the grid ionospheric vertical error (GIVE). The intersection of the line of sight from receiver to satellite and the shell defined by the IGPs is known as the user's ionospheric pierce point (IPP). A user interpolates the grid node delay to the IPP to obtain an estimate of the ionospheric delay at the IPP. However, fluctuations in the characteristics of the ionosphere during the day and from day to day can cause significant errors in the delay estimations at the user IPPs.
Plasma depletions are strong reductions in the ionosphere F-region plasma density due to the appearance of a Rayleigh-Taylor instability, and produce significant reduction in the above mentioned ionospheric delay crossing the depletion zone. Plasma depletions are mostly aligned to the geomagnetic field and confined to the Appleton Anomaly region. As a result, equatorial regions are particularly affected by plasma depletions, with plasma density decreases of up to about three orders of magnitude (99.9%). However, plasma depletions are small-scale phenomena, generally seen in only one satellite-user line-of-sight. As a result, and due to the limited number of reference stations, depletions are not easily nor reliably sampled by SBAS systems which generally provide large-scale ionospheric information.